The present invention relates to apparatus and methods for generating a synthetic image, and more particularly relates to a system for combining information generated in response to two different detectable characteristics to produce an enhanced color output image.
In general, a digital image contains data that is generated by a transducer responsive to a particular detectable characteristic of a scene. If the data generated by the transducer is manipulated, for example to enhance the quality of the image, the resultant image is generally referred to as a synthetic image. By way of example, a night vision system might gather light from a scene and generate a synthetic image of the scene, using processing to enhance certain features of the image to improve its utility.
Night vision systems are useful in a variety of applications, such as in automated target identification systems and in night vision scopes which are used, for example, by law enforcement and military agencies for both surveillance and mobility purposes. For example, a helicopter pilot following a road through a forest at night requires a night vision system that is capable of distinguishing the road from the forest canopy. Further, in order to permit safe operation of the helicopter, the night vision system must also reliably detect changes in elevation of the forest canopy. Similar needs for enhanced night mobility and targeting are relevant to the soldier on the ground, on foot or in vehicles.
As is well known in the night vision arts, data gathered from different sources, such as from transducers responsive to light from different optical spectral ranges representative of the same scene, provide different and complementary types of visual information. Continuing the previous night flying helicopter example, a road is typically a good source of Long Wave Infrared (LWIR or thermal infrared) light at night while a forest is not. Consequently LWIR sensors are particularly useful for locating a road within a forest. Since the temperature of a forest does not change rapidly with changes in elevation, LWIR sensors are not particularly useful for locating hills and mountains in a forest. However, a forest canopy is a relatively good reflector of Visible through Near-Infrared (VIS-NIR) reflected light, which may be caused by starlight, while a road is not. Consequently, sensors which detect low level VIS-NIR reflected light can be particularly useful for locating hills and mountains, but are not particularly useful for locating a road within a forest.
Scopes which allow a pilot to switch between a LWIR sensor and a VIS-NIR sensor, and systems which present the output of both types of sensors on two different monitors, are known, but those systems are generally of limited utility because they require too much effort on the part of the pilot to integrate the data.
There is therefore a need for a system that combines information from two different sensor sources and generates a single image, i.e., a synthetic image, that reliably presents the data from the different sensors. Further, the system must combine the data in a meaningful way so that a human observer can easily absorb the data.
Another problem with known night vision scopes relates to the dynamic range compression that is necessary to display an output image on a small inexpensive monitor which typically has a much smaller dynamic range than the commonly used light sensing elements, such as CCDs or infrared cameras. In the prior art, dynamic range compression is typically achieved through a global scaling which although is computationally simple, generally yields inadequate performance. Global scaling generally involves transforming the maximum and minimum values of the sensor output to the brightest and darkest points, respectively, that the monitor is capable of displaying, and linearly transforming all other sensor output values to points between these extreme bright and dark points. This type of scaling insures that the output image is displayed using the full dynamic range of the monitor, however it also typically results in the loss of much rich contrast information. For example, in an infrared image of a scene that is mostly cool and has one hot spot, global scaling typically compresses all contrast information in the cool regions to the same uniform dark value for the sake of displaying the single hot spot. Similar problems arise when displaying a low-light visible image of a scene with wide intra-scene dynamic range.
There is therefore a need for a night vision system having local automatic gain control such that it generates output images which preserve useful contrast information in both the bright and dark regions of the display.
Accordingly, it is an object of the invention to provide an improved method and apparatus for combining visual information from two different sources into a single output image.
It is a further object of the invention to provide a night vision system having automatic gain control for generating output images that preserve contrast information in both the bright and dark regions of the display.
Other objects and advantages of the present invention will become apparent upon consideration of the appended drawings and description thereof.